1. Field of the Invention
The present invention relates to a hard carbon film that comprises graphite-like diamond and carbon clusters and to a surface-acoustic-wave (SAW) substrate that comprises a base material, the hard carbon film mounted on the base material, and a piezoelectric layer mounted on the hard carbon film.
The hard carbon film of the present invention is suitable for wide use in electronic parts and electronic materials in general, such as a low-cost heatsink for mounting lasers on it, and the SAW substrate of the present invention is particularly suitable for use in SAW devices.
2. Description of the Background Art
Lithium niobate (LNO), lithium tantalate (LTO), crystal, sapphire, and ZnO have been used as the material for SAW substrates. Single-crystalline diamond, C-BN, AlN, Cu, and Al have been used as the material for heatsinks.
With the material for SAW use, the fabrication of high-frequency SAW filters by the use of a conventional material, such as crystal, requires the technology for miniscule-wiring of 1 xcexcm or below, especially sub- xcexcm or below, causing a serious problem of yield reduction resulting from breaks and short circuits in wiring. In addition, the implementation of the miniscule-wiring technology requires the provision of a large-scale clean room and a large volume of investment.
A highly crystalline diamond allows easy development of surface unevenness caused by the protrusion of the crystal""s (111) and (100) facets, making it extremely difficult to obtain surface evenness by polishing. The diamond is prone to grow to a grain size of 5 xcexcm or more, which is responsible for the propagation loss when a high-frequency device is fabricated on the diamond surface.
SAW devices for 1.0 GHz or higher with a piezoelectric-body/diamond laminated structure have a problem that they cause comparatively great propagation loss when used for a device such as a filter, although their operating frequency is on a satisfactory level. At such a high frequency, the wavelength becomes short and thereby increases the effect of microscopic surface unevenness of the piezoelectric body on the propagation loss and enlarges the scattering of surface waves at grain boundaries. Since diamond is harder than any other substance, it is difficult to obtain a film with sufficient surface evenness.
Diamond-like carbon, which transmits sound waves at a speed comparable to that of diamond, allows easy production of an even film. However, it permits facile evaporation of carbon into vacuum during the formation of a piezoelectric body, so that devices comprising the carbon have been difficult to fabricate.
With the material for heatsinks, ultra-high-pressure single-crystalline diamonds are high in cost, although they have high thermal conductivity. Consequently, adequate heat conductive materials have been required which are low in cost or allow easy fabrication.
At present, it is difficult to obtain single-crystalline diamonds by chemical vapor deposition (CVD). Inevitably, diamond films obtained by CVD are polycrystalline films, which have grain boundaries. Because the grain boundaries scatter surface waves, SAW filters comprising the diamond films obtained by CVD have tended to deteriorate in performance.
An object of the present invention is to offer a hard carbon film and a SAW substrate that are free from the drawbacks such as those of the above-described prior art.
Another object of the present invention is to offer a hard carbon film and a SAW substrate that are easy to fabricate and are low in manufacturing cost while virtually maintaining the quality that affects the important properties of a device that comprises the hard carbon film or the SAW substrate.
Yet another object of the present invention is to offer a hard carbon film and a SAW substrate that are low in propagation loss when used in SAW devices.
The present inventors, through intensive experiments and studies, have found that a graphite-like diamond/carbon-clusters composite film that has a specific characteristic is highly effective to achieve the above-described objects.
The hard carbon film of the present invention is based on the above finding. More specifically, it comprises a composite film of graphite-like diamond and carbon clusters, and the composite film has a continuous crystal structure.
The present inventors consider that the hard carbon film of the present invention having the above-described constitution is composed of graphite-like diamond constituting the bulk, desirably 90% or more, of the hard carbon film and carbon clusters filling the interstices between the diamond grains.
Conventional SAW filters have allowed the scattering of surface waves at the diamond grain boundaries as an inevitable consequence, causing the propagation loss. On the other hand, the hard carbon film of the present invention has a low propagation loss comparable to the level that has been considered to be attainable only by a single-crystalline diamond (not attainable by polycrystalline diamonds). This reduction in propagation loss is attributed to the structure of the hard carbon film; i.e., the continuity of the crystal structure is maintained at a level comparable to that in a single crystal because the diamond crystal grains are similar to graphite and the interstices between the diamond grains are filled with carbon clusters.
The hard carbon film of the present invention has a level equivalent to diamond in terms of physical properties and hardness, and therefore transmits SAWs at a speed comparable to that in high-quality diamonds. The hard carbon film facilitates the attainment of a highly even surface, 1 nm or less in surface roughness (Ra), by polishing with a simple diamond grinding wheel because it has few (111) planes of diamond in a plane parallel to the base material and because its crystals are not perfect diamond cystals.
Consequently, the hard carbon film of the present invention, when used in SAW filters, facilitates the growth of a highly flat, high-quality film of a piezoelectric body, which is an essential member of a SAW filter, on the hard carbon film.
As described above, the present invention offers a hard carbon film that comprises a composite film of graphite-like diamond and carbon clusters, the composite film having a continuous crystal structure.
When analyzed by Raman spectroscopy, the hard carbon film of the present invention has a diamond peak of which the peak value lies at a Raman shift of 1332 cmxe2x88x921 or more, desirably in the range of 1333 to 1335 cmxe2x88x921. The peak has a full width at half maximum (FWHM) of 6 cmxe2x88x921 or more. The hard carbon film has only one carbon-cluster peak at a Raman shift of 1500 to 1520 cmxe2x88x921 in the range of 1400 to 1700 cmxe2x88x921. The peak has an FWHM of 170 cmxe2x88x921 or less. The hard carbon film further has an Ic/Id ratio of 4 or more, where Ic means the integrated intensity of the carbon-cluster peak and Id means the integrated intensity of the diamond peak in the Raman spectrum.
The present inventors consider that the hard carbon film of the present invention having the above-described Raman spectroscopic properties has a composite structure in which the bulk, 90% or more, for example, of the hard carbon film is constituted by graphite-like diamond, and the interstices between the diamond grains are filled with carbon clusters.
The present inventors found that the hard carbon film of the present invention having the appropriate percentage of the carbon clusters as described above facilitates the formation of a highly even mirror-finished surface by polishing the film.
The conventional multicrystalline-diamond films have conspicuous grain boundaries. The interstices between these diamond grains are either vacant or filled with graphite. Even in the latter case, the discontinuity of the crystal structure is inevitable. Therefore, when these multicrystalline-diamond films are used in SAW filters, surface waves are scattered at the grain boundaries. As a result, the films are not suitable for use in the substrates for SAW filters. On the other hand, when the hard carbon film of the present invention is used as a component of the substrate for a SAW device, the decrease in SAW propagation velocity can be prevented without the practical reduction in the surface hardness of the hard carbon film because of the above-described properties of the hard carbon film.
As described above, the present invention offers a hard carbon film that is easy to fabricate or low in manufacturing cost while virtually maintaining the quality that affects the important properties of a device that comprises the hard carbon film. The hard carbon film has the great advantage of low propagation loss when used in the substrate for SAW devices in particular.